Electronic timer



y 1963 l. s. YAVELBERG 3,088,409

ELECTRONIC TIMER Filed Nov. 28, 1960 ARMING MECHANISM We IZQ:

l l l l sauna E B2 7 20 :1? I] l6 m i B FIG. 2.

JNVEMTOR, mvm s. Y'AVELBERG v ATTO KIM 3,088,409 ELECTRONIC TIMER IrvinS. Yavelberg, Tucson, Ariz., assignor to the United the Secretary of Theinvention described herein may be manufactured and used by or for theGovernment of the United States of America for governmental purposesWithout the payment of any royalties thereon or therefor;

The present invention relates to timing means, and particularly to asquib-firing electronic timing apparatus.

While adaptable for use in many different types of equipment, theinvention is of special utility in missile destructor fuze apparatus andwill be described principally with reference to such apparatus.

It is often of importance that air-to-air or ground-toair missiles beprevented from endangering objects beyond the target, or property andpersonnel on the ground below, and for this reason such missiles mustinclude means operative to accomplish self-destruction of the missilewhile still aloft and at a predetermined distance beyond the launchingpoint or, more conveniently, at a predetermined time after the launchinginstant, in the event the missile has passed its intended target. Theself-destruction means requires an explosive charge (conveniently, thatalready carried in the warhead), and a missile destructor fuze apparatuspreferably including a destructor squib and timing means to effectelectrical detonation or flashing of the destructor squib. Simplicity,sturdiness, compactness and low Weight being at a premium in missiledesign, it is particularly advantageous to employ a timing device ofunijunction transistor type which makes it possible to achieve suchcharacteristics, provided that such a timing device can also be made toyield substantially constant timing (for any selected setting) over thewide temperature range called for by military specifications.

Unijunction transistor type of timing devices make use of an integratingcircuit wherein the voltage developed across a capacitor as a functionof charging time reaches a transistor firing value upon expiration of apredetermined charging interval, hereinafter termed the squibflashingdelay-time. For use in missile self-destruction apparatus as indicated,the squib-flashing delay-time, as measured from substantially themissile launch instant, must be comparatively long, say 60 seconds byway of example. The integrating circuit, further, must be so designedthat sufficient energy will have accumulated in the charging capacitor,at the intended self-destruction instant, to effect flashing of thedestructor squib. It therefore becomes impractical to employ a chargingcapacitor of any type other than an electrolytic capacitor ofcomparatively large microfarad value, preferably a tantalum capacitorbecause of its extreme compactness. It has been found, however, thatover the typical military specification range of temperatures, say -50to +150 on the Fahrenheit scale, the change in leakage resistance (andcorrespondingly of leakage current), of a tantalum capacitor asheretofore employed in timing devices broadly of the type hereunderconsideration, may be so large as to result in delay-time variations ofthe order of 100 percent, far too great to be tolerated in missiledestructor fuze apparatus.

It is therefore the principal object of the present invention to providea missile destructor fuze apparatus having improved reliability.

It is another object of the invention to provide a unijunctiontransistor timing device providing a high degree assess Patented May 7,1963 of timing stability over a relatively wide range of operatingtemperatures.

Other objects and many of the attendant advantages of this inventionwill be readily appreciated as the same becomes better understood byreference to the following detailed description when considered inconnection with the accompanying drawings wherein:

FIG. 1 is a schematic diagram of a unijuncion transistor timing circuitin accordance with principles of the invention; and

FIG. 2 is a schematic diagram of a modified unijunction transistortiming circuit employing a single DC. power source.

Referring now to FIG. 1, the unijunction transistor 10 may be ofconventional type consisting of a small bar of uniformly doped N-typesilicon having ohmic contacts identified as B1 (base-one) and B2(base-two) at its two ends, and a single rectifying PN junctionidentified as emitter E. Unijunction transistor 10 presents a conductioncontrol feature somewhat similar to that of a gas thyratron. Inparticular, until a control voltage of positive polarity applied toemitter E assumes a critical threshold value of the order of six-tenthsof the inter-base voltage, emitter E is in eifect reverse-biased, andthe unijunction transistor is said to be cut off, essentially so exceptfor a so-called reverse or back current of comparatively small magnitudein the emitter circuit. When the critical threshold value of controlvoltage is reached, however, the emitter E becomes forward-biased, andthe resistance between emitter electrode E and base electrode B1 of thetransistor drops sharply to a comparatively low value, enablingdestructor squib 11 to be flashed, as will appear.

Separate DC. power sources 12 and 13 are provided, in the FIG. 1embodiment, for supplying the unijunction transistor base-to-basevoltage, and for supplying capacitor charging current to the integratingcircuit comprising adjustable resistor 14, and capacitors 15 and 16having substantially the same capacitance and leakage currentcharacteristics. These power sources 12 and 13 are most practically andconveniently provided in the form of batteries, as indicated, and thepower circuits are arranged to remain open until actuation of switches17 and 18. Switches 17 and 18, forming part of anacceleration-responsive mechanism 19 which may be of any conventionaltype (therefore not shown in detail) such as employed for missilearming, are to be understood as being thrown into closed condition bymechanism 19, to start the timing function of the unijunction transistorcircuit, provided the missile functions successfully to execute rapidacceleration. Relative to the instant at which the switches 17 and 18are closed, the control voltage developed across capacitor 16 reachesthe firing threshold value after a so-called delay-time dictated by theohmic value of resistance provided by adjustable resistor 14, themicrofarad values of capacitors 15 and 16, and the magnitudes of thevoltages delivered by sources 12 and 13. It has been found that withcompensating capacitor 15 and firing capacitor 16 of exactly the sametype and selected to exhibit the same values of capacitance and leakageresistance at normal temperature and, correspondingly, likecharacteristics of capacitance and leakage resistance variations withtemperature change, the timing function for any given setting ofresistor 14 remains reasonably constant over the rather wide militaryspecification temperature range indicated above. For example, a testversion of the FIG. 1 circuit, employing a unijunction transistor ofcommercially available 2N489 type, sources 12 and 13 respectivelydelivering 15 and 30 volts, tantalum capacitors each of microfarad valueand rated at say 30 volts, and a series charging resistance of 1.4megohms, has exhibited a squib-flashing delay-time of approximately 60seconds with variations of only about 3 5 percent over the temperaturerange of 50 F. to +170 F.

FIG. 2 concerns a modification providing like timing stability, butpresenting further improvement as to compactness and low weight by useof a single tapped battery comprising sections 20 and 21, and acompensating resistor 22, as illustrated. Battery section 20 serves asthe source of interbase voltage for unijunction transistor 10, andbattery sections 20 and 21 are additive as to the charging voltageapplied to the series circuit of resistor 14 and capacitors 15 and 16.Near-optimum battery compactness is provided by selection of batterysection to a yield voltage of about four-tenths of the additive voltage.Resistor 22 is of suitable ohmic value to impose a current drain ofabout the same magnitude on battery section 21 as is imposed uponbattery section 20 by unijunction transistor 10, in this manner servingto maintain substantially the same ratio of terminal voltages suppliedby the battery sections under varying conditions, as necessary tomaintain the same timing function for a given setting of resistor 14.

Firing or flashing of squib 11 in the FIG. 1 and FIG. 2 circuits iseffected by discharge of capacitor 16 through squib 11, enabled at theinstant when the control voltage developed in the capacitor has reachedthe critical value, sometimes termed the firing or peak point emittervoltage, which establishes a forward-bias condition for emitter E. In atypical instance squib 11 may present a resistance of approximately 5ohms and may require about 5000 ergs of electrical energy to flash it.Assuming that the peak point emitter voltage required to fire theunijunction transistor 11 is of the order of say 10 volts, the energyaccumulated in capacitor 16 at that time would be 50,000 ergs, wellabove the squib-flashing energy requirements in order to insure strongflashing and explosive ignition action.

While the FIG. 1 and FIG. 2 circuits provide stability of squib-flashingdelay-time by means of a pair of seriesconnected like capacitors havingcompensating leakage current characteristics, for use in anon-adjustable or narrowly adjustable timer intended to providesubstantially fixed rather than widely variable delay-time, it is alsofeasible to employ a single capacitor, in place of the pair ofseries-connected capacitors, presenting capacitance and leakageresistance characteristics which vary oppositely with temperature andcombine to yield the necessary transistor-firing control voltage atsubstantially constant delay-time over the specified operatingtemperature range.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

What is claimed is:

1. A missile destructor fuze apparatus comprising, in

combination: a destructor squib; a unijunction transistor having anemitter electrode and a pair of base electrodes; means for providing apredetermined potential gradient in said unijunction transistor betweensaid base electrodes and correspondingly establishing a predeterminedvalue of peak point emitter voltage for firing said unijunctiontransistor; a series-charging network comprising a source of voltage, aswitch, a resistor, a compensating tantalum capacitor, and a firingtantalum capacitor, to effect charging of said firing capacitor to saidpredetermined value of peak point emitter voltage upon expiration of apredetermined time delay initiated upon closing of said switch; saidfiring capacitor, squib, and unijunction transistor being connected incircuit to effect flashing of said squib by conduction of dischargecurrent from said firing capacitor through said squib and said emitterelectrode when the firing capacitor voltage reaches said predeterminedvalue of peak point emitter voltage; and said capacitors havingbalancing temperature-dependent characteristics which render saidpredetermined delay time substantially independent of ambienttemperature changes.

2. A missile destructor fuze apparatus comprising, in combination: adestructor squib; a unijunction transistor having an emitter electrodeand a pair of base electrodes; a battery having a section thereofconnected across the base electrodes of said unijunction transistor toprovide a predetermined potential gradient therebetween andcorrespondingly establishing a predetermined value of peak point emittervoltage for firing said unijunction transistor; a compensatingcurrent-drain resistor connected across the remaining section of saidbattery; a series-charging network comprising said battery, a switch, aresistor, a compensating tantalum capacitor, and a firing tantalumcapacitor, to effect charging of the firing capacitor to saidpredetermined value of peak point emitter voltage upon expiration of apredetermined time delay initiated upon closing of said switch; saidfiring capacitor, squib and unijunction transistor being connected incircuit to effect flashing of said squib by conduction of dischargecurrent from said firing capacitor through said squib and said emitterelectrode when the firing capacitor voltage reaches said predeterminedvalue of peak point emitter voltage; and said capacitors havingbalancing temperature-dependent characteristics which render saidpredetermined delay time substantially independent of ambienttemperature changes.

References Cited in the file of this patent UNITED STATES PATENTS2,906,206 Morison et al. Sept. 29, 1959 FOREIGN PATENTS 1,136,465 FranceDec. 29, 1956 1,218,809 France Dec. 21, 1959

1. A MISSILE DESTRUCTOR FUZE APPARATUS COMPRISING, IN COMBINATION: ADESTRUCTOR SQUIB; A UNIJUNCTION TRANSISTOR HAVING AN EMITTER ELECTRODEAND A PAIR OF BASE ELECTRODES; MEANS FOR PROVIDING A PREDETERMINEDPOTENTIAL GRADIENT IN SAID UNIJUNCTION TRANSISTOR BETWEEN SAID BASEELECTRODES AND CORRESPONDINGLY ESTABLISHING A PREDETERMINED VALVE OFPEAK POINT EMITTER VOLTAGE FOR FIRING SAID UNIJUNCTION TRANSISTOR; ASERIES-CHARGING NETWORK COMPRISING A SOURCE OF VOLTAGE, A SWITCH, ARESISTOR, A COMPENSATING TANTALUM CAPACITOR, AND A FIRING TANTALUMCAPACITOR, TO EFFECT CHARGING OF SAID FIRING CAPACITOR TO SAIDPREDETERMINED VALUE OF PEAK POINT EMITTER VOLTAGE UPON EXPIRATION OF APREDETERMINED TIME DELAY INITIATED UPON CLOSING OF SAID SWITCH; SAIDFIRING CAPACITOR, SQUIB, AND UNIJUNCTION TRANSISTOR BEING CONNECTED INCIRCUIT TO EFFECT FLASHING SAID SQUIB BY CONDUCTION OF DISCHARGE CURRENTFROM SAID FIRING CAPACITY THROUGH SAID SQUIB AND SAID EMITTER ELECTRODEWHEN THE FIRING CAPACITOR VOLTAGE REACHES SAID PREDETERMINED VALVE OFPEAK POINT EMITTER VOLTAGE; AND SAID CAPACITORS HAVING BALANCINGTEMPERATURE-DEPENDENT CHARACTERISTICS WHICH RENDER SAID PREDETERMINEDDELAY TIME SUBSTANTIALLY INDEPENDENT OF AMBIENT TEMPERATURE CHANGES.